Analogue digital converters, abbreviated to ADC, have many varied applications in electronic signal processing. The outputs of sensors are most commonly available as analogue signals, and must first be appropriately converted before further digital processing. A problem that frequently occurs at this stage is that the output voltages from the sensors are larger than the range of input voltages suitable for analogue/digital converters implemented in conventional semiconductor technologies. These are constructed on integrated circuits employing, for instance, bipolar, unipolar or a mixed technology such as BiCMOS, and usually have permitted voltage ranges of only a few volts.
In order to the able to handle signals with greater amplitudes, the analogue/digital converter can be built in what is known as a high-voltage technology using, in other words, an integrated circuit technology that, as a result of appropriate measures, can withstand higher voltages of, for instance, 10 volts or more. Alternatively, the sensor's output signal can be sufficiently attenuated that it lies within the permitted range of input voltages of a conventional ADC.
A disadvantage to making ADCs in high-voltage technology is that the conversion rates are significantly lower than is the case for conventional low-voltage ADCs. The reason for this is that, in order to achieve the higher breakdown voltages, high-voltage transistors have larger dimensions and are therefore slower.
Attenuation of the signal by means, for instance, of a resistive voltage divider at the input to the ADC in order to transform an input signal with a greater amplitude down to one within the range of voltage permitted for the ADC, has the disadvantage that the voltage divider presents an ohmic load to this source at the input of the ADC, and also that the current consumption of the ADC is greater, since a current path through the voltage divider is always available.
Document U.S. Pat. No. 6,731,232 B1 describes an ADC that operates according to the principle of successive approximation. In order to achieve a programmable input voltage range, a high-voltage sampling switch is provided at the input. This is not preceded by any additional attenuator circuits. This permits the input voltage to be directly sampled on one or more sampling capacitors. The analogue input can be scaled or attenuated in order to match the dynamic range of the ADC. This allows the processing of voltages larger than the permitted input voltages found on conventional integrated circuit technologies, which may also be referred to as low-voltage technologies. As is shown, for instance, by FIG. 4 of the US document mentioned, a reference voltage Vcom, to which all the capacitances in the AD converter are connected, is required for operation of the comparator. To ensure adequate drive, the Vcom signal needs to be buffered by an amplifier. This, in turn, entails increased current consumption by the ADC. As is shown in FIG. 2 of the document cited, a large number of high-voltage switches are required at the input to the ADC, and these, in turn, have a large space requirement. In addition, voltages can occur at the comparator input that are larger than the permitted low-voltage range. The reason for this is that, at the lowest programmed input voltage range, a voltage present at the input, which is smaller than the high supply voltage but significantly larger than the permitted input voltage of the ADC, is indeed attenuated by the voltage divider, but input voltages at the comparator occurring during the successive approximation can damage the gate oxide of its input transistors or can break it down.